Ball valve seat



FIG. 1 is a left-front perspective view of the Design for the ball valve seat;

FIG. 2 is a right-front perspective view thereof;

FIG. 3 is a left view thereof;

FIG. 4 is a right view thereof;

FIG. 5 is a front view thereof, the rear, the top, and the bottom views thereof being the same as the front view thereof; and,

FIG. 6 is a cross-sectional view thereof taken along line 6-6 of FIG. 4. 

CLAIM The ornamental design for a ball valve seat, substantially as shown and described. 